Silicon carbide (SiC) is a thermally and chemically stable compound semiconductor. Compared with silicon (Si), SiC has an excellent band gap, break down voltage, electron saturation velocity, and thermal conductivity. Therefore, SiC is promising for application in technical fields such as power device materials with low operating loss, high-voltage resistant high-frequency device materials, environment-resistant devices used in high-temperature environments, and radiation-resistant devices. In these technical fields, a high quality SiC single crystal with few crystal defects is demanded.
Methods for manufacturing a SiC single crystal include a sublimation method and a solution growth method. Compared to the sublimation method, the solution growth method can produce a Sic single crystal with less crystal defects. The solution growth method of a SiC single crystal is a method of growing a SiC single crystal by bringing a SiC seed crystal into contact with a solution in which carbon (C) is dissolved into a melt containing Si or Si and additive elements (hereafter referred to as a SiC solution). In general, a crucible made of graphite is used, and carbon dissolves from the crucible to be fed into the SiC solution. The SiC seed crystal is attached to a lower end surface of a bar-shaped seed shaft and is brought into contact with the SiC solution.
In the solution growth method, the temperature of the portion of the SiC solution in the periphery of the SiC seed crystal (hereafter, referred to as a SiC seed-crystal peripheral area) is kept lower than that of the other portion of SiC solution. This will cause a supersaturation of SiC in the SiC seed-crystal peripheral area, thereby accelerating the growth of the SiC single crystal.
As described above, the solution growth method allows manufacture of a SiC single crystal which has less crystal defects than in the sublimation method. However, the growth rate of the SiC single crystal in the solution growth method is lower than in the sublimation method. For example, in a conventional solution growth method, the growth rate of SiC single crystal in solution growth method is about 5 to 12 μm/hr when a pure Si solution of 1650° C. is used. This growth rate is not more than 1/10 of that of SiC single crystal in the sublimation method.
The growth rate RA (m/s) of SiC single crystal in the solution growth method is defined by the Wilson-Frenkel formula shown in Formula (A):RA=A0×ΔC×exp(−ΔG/(k×t))  (A)
where, A0 is a coefficient, ΔC is the degree of supersaturation of carbon (unit: mol/m3), ΔG is the energy (unit: J/mol) needed for taking away a solvent molecule from a solute molecule, k is the gas constant (unit: J/K·mol), and t is absolute temperature (K).
It can be seen from Formula (A) that increasing the degree of supersaturation (ΔC) of carbon in the SiC solution allows an increase in the growth rate RA of SiC single crystal. Increasing the feed rate of carbon to a portion near the SiC seed crystal, of the SiC solution will allow an increase in the degree of supersaturation of carbon (ΔC).
JP2006-117441A (Patent Document 1) discloses a technique to increase the feed rate of carbon to the vicinity of a SiC seed crystal. Patent Document 1 applies an accelerated crucible rotation technique (hereafter, referred to as ACRT method) to the solution growth method of SiC single crystal. In the ACRT method, acceleration and deceleration are repeated for the rotation of the SiC seed crystal and the rotation of the crucible. This will cause the SiC solution to be stirred, thereby facilitating feeding of carbon to the vicinity of the SiC seed crystal.